In this type of writing implement, the reservoir containing the ink is not Isolated from ambient air, with air reaching the reservoir via the transfer rod. As a result, any variation in conditions of use, and in particular an increase in the pressure of the air contained in the reservoir due to said air heating, gives rise to an abnormal flow of ink along the transfer rod, which flow can give rise to smudges or blobs when the cap is taken off the implement. The function of the buffer reservoir is to absorb excess ink coming from the reservoir before it reaches the writing tip, i.e. the front end of the transfer rod which is used for writing purposes, and then, once normal conditions have been reestablished, of giving the ink it contains back to the transfer rod. Various embodiments of the buffer reservoir have already been proposed. In document EP 380 696, the buffer reservoir is a molded part having radial slots in the form of a comb and longitudinal slots putting said radial slots into communication with the hollowed-out central portion that is in contact with the transfer rod.
In document CH A 422 575, the buffer reservoir is a spongy mass of open cells, preferably made of a material that does not absorb the ink.
In document WO 92/20530, the structure of the buffer reservoir is defined merely as being a capillary storage element having capillarity that is, on average, less than the average capillarity of the transfer rod, at least at the junction orifice between the transfer rod and the tank of liquid ink.
The buffer reservoir is made of porous and/or fibrous material.
In document EP 516 538, the buffer reservoir is in the form of at least one compact block that fits snugly around the transfer rod. The buffer reservoir may include a porous hydrophobic material based on microspheres obtained by thermofusion of microspheres of one or more thermoplastic materials, and in particular microspheres of polypropylene or of polyethylene.
Writing implements using liquid ink and a buffer reservoir of the kind described above have given rise to applications only in the field of aqueous inks designed for writing on absorbent media.